Improving the sensitivity of piezoelectric composites by adding porosity

Abstract

Piezoelectric materials have the ability to convert mechanical to electrical energy (direct effect) and vice versa. They are readily used in the aerospace, automobile, telecommunication industry etc. as both sensors and actuators. For this work the focus is on the sensor application, which utilizes the direct piezoelectric effect. With the rapidly growing technological demands, sensors should be flexible enough to adapt to different applications while also having adequate sensing capabilities. Currently, lead- and lead-based piezoelectrics are used in the industry due to their excellent piezoelectric properties. However, due to their toxic nature, research has been ongoing into more lead-free systems which are capable of replicating the performance of these lead-based systems. In this work, we aim to improve the sensing capabilities of lead-free piezoelectric composites. To further improve their performance, reducing the dielectric constant (ε) of the composite is the main strategy of this work. The reduction is achieved by fabricating a porous composite structure. The reduction in permittivity leads to an increase in the piezoelectric voltage constant (g), which defines the sensitivity of the piezoelectric composite.

The main focus of this work is to optimize a polymer and polymer foaming technique to obtain a high level of porosity, while also retaining adequate mechanical properties. The next step is to achieve a high poling efficiency for the composite in order to obtain good piezoelectric properties. For the polymer system, polyvinyl alcohol (PVA) is selected as the matrix due to its excellent film forming ability as well as its relatively high dielectric properties (compared to polymers). The direct foaming technique is used for this work, due to its simplicity and its reproducibility. For the lead-free ceramic system, Barium Titanate (BaTiO3) and Sodium Potassium Niobate doped with Lithium (KNLN3) is selected as they have good piezoelectric properties, and have been used in piezoelectric composites extensively. As a porous piezoelectric composite is used in this work, the contact poling is replaced by the corona poling method to prevent localized dielectric breakdowns and non-uniform poling.

With the direct foaming technique, foams with porosites in the range of 90-95 % are obtained, resulting in a drastic reduction in the permittivity of the composite. Such a high porosity level also results in a much softer composite. The optimization of the corona poling process is done by selecting the adequate poling temperature and the grid voltage, which is found to be 110 °C and 6 kV respectively. The effective piezoelectric charge coefficient is measured using Al plates as electrodes, to prevent the soft composites from compressing locally. The foam composites exhibit remarkably high g33 values exceeding the 1000 mV.m/N mark, almost double the best sensor used in the industry currently (PVDF). This is attributed to the high poling efficiency and the reduced dielectric permittivity of the composite. This opens up the vast number of possibilities for future systems based on porous structures to be used as sensors which can showcase good piezoelectric properties as well as being more flexible/conformable.